Such energy storage and power generation systems have been known for many years. Major limitations of these systems have resulted from the practical application of what seems to be a simple direct chemical process. Hazardous materials, efficiencies, system size, plugging and clogging, gas formation, "plating out" or precipitation of the materials, membrane diffusion limitations, cost of materials and cost of operation highlight the practical problems. Another limitation of such systems is the loss of power output as the system discharges.
The fundamental chemical process in these systems is characterized by a chemical equation where the action proceeds in one direction in the charging of the system and in the opposite direction during the power generation by the system. An example of a redox system is given by the following chemical equation, the term "redox" defining reactions in which a reduction and a complementary oxidation occur together. EQU Cr.sup.2+ +Fe.sup.3+ .revreaction.Cr.sup.3+ +Fe.sup.2+ Eq. 1
In this system, limitations exist since the chromium is expensive, and the chromium and iron, meant to be on either side of a membrane, cross over contaminating the other side. This necessitates frequent reprocessing of the electrolyte. Furthermore, noble metal catalysts are required to promote the reaction. Also, the system pH must be controlled to prevent gas formation.
U.S. Pat. No. 4,485,154 discloses an electrically chargeable anionically active reduction-oxidation system using a sulfide/polysulfide reaction in one half of the cell and an iodine/polyiodide, chlorine/chloride or bromine/bromide reaction in the other half of the cell.
The overall chemical reaction involved, for example for the bromine/sulfide system is EQU Br.sub.2 +S.sup.2- .revreaction.2Br.sup.- +S Eq. 2
The electrochemical reaction takes place in separate but dependent bromine and sulfur reactions. The bromine reaction takes place on the +.sup.ve side of the membrane and the sulfur reaction on the -.sup.ve side of the membrane. When charging occurs the reaction goes from right to left and when discharging the reaction goes from left to right. During extended cycling of the cell ionic species diffuse through the membrane in an unwanted direction. Some sulfide diffuses into the +.sup.ve side of the cell and some of this sulfide is oxidised by the bromine in the +.sup.ve side to the sulfate SO.sub.4.sup.2-. Sulfates are not readily retrievable from the +.sup.ve electrolyte and thus represent a net loss of sulfur from the system.
Another reduction-oxidation system using a sulfide/polysulfide reaction in one half of the cell combines this reaction with an Fe.sup.3+ /Fe.sup.2+ couple according to the following overall chemical equation: EQU 2Fe.sup.3+ +S.sup.2- .revreaction.2Fe.sup.2+ +S Eq. 3
The electrochemical reaction takes place in separate but dependent iron and sulfur reactions. The iron reaction takes place on the +.sup.ve side of the cell and the sulfide reaction on the -.sup.ve side of the cell. In this system, a cell containing only two compartments will not accept a charge if charge carrying ions are not present in the +.sup.ve chamber during charging of the cell. Attempting to charge such a cell results in an immediate high resistance and no current flows. This is because any iron ions travelling across the membrane react with the S.sup.2- ions to form an iron sulfide precipitate which clogs the pores of the membrane.
I have now developed an electrochemical apparatus for energy storage and/or power delivery which prevents or reduces the deposit of solids on the electrodes or the membranes, and which enables the electrolytes to be managed by reducing the formation of unwanted species or the formation of species which will interfere in the cell reaction.